Geomorphic evidence for the geometry and slip rate of a young, low-angle thrust fault: Implications for hazard assessment and fault interaction in complex tectonic environments

We present surface evidence and displacement rates for a young, active, low-angle (∼20°) reverse thrust fault in close proximity to major population centers in southern California (USA), the Southern San Cayetano fault (SSCF). Active faulting along the northern flank of the Santa Clara River Valley displaces young landforms, such as late Quaternary river terraces and alluvial fans. Geomorphic strain markers are examined using field mapping, high-resolution lidar topographic data, ¹⁰Be surface exposure dating, and subsurface well data to provide evidence for a young, active SSCF along the northern flank of the Santa Clara River Valley. Displacement rates for the SSCF are calculated over 10³–10⁴ yr timescales with maximum slip rates for the central SSCF of 1.9[Formula presented] mm yr⁻¹ between ∼19–7 ka and minimum slip rates of 1.3[Formula presented] mm yr⁻¹ since ∼7 ka. Uplift rates for the central SSCF have not varied significantly over the last ∼58 ka, with a maximum value of 1.7[Formula presented] mm yr⁻¹ for the interval ∼58–19 ka, and a minimum value of 1.2±0.3 mm yr⁻¹ since ∼7 ka. The SSCF is interpreted as a young, active structure with onset of activity at some time after ∼58 ka. The geometry for the SSCF presented here, with a ∼20° north dip in the subsurface, is the first interpretation of the SSCF based on geological field data. Our new interpretation is significantly different from the previously proposed model-derived geometry, which dips more steeply at 45–60° and intersects the surface in the middle of the Santa Clara River Valley. We suggest that the SSCF may rupture in tandem with the main San Cayetano fault. Additionally, the SSCF could potentially act as a rupture pathway between the Ventura and San Cayetano faults in large-magnitude, multi-fault earthquakes in southern California. However, given structural complexities, including significant changes in dip and varying Holocene displacement rates along strike, further work is required to examine the possible mechanism, likelihood, and frequency of potential through-going ruptures between the Ventura and San Cayetano faults. Confirmation of the SSCF in a previously well-studied area, such as southern California, demonstrates that identification of young faults is critical for accurate seismic hazard assessment. We suggest that many young, active faults remain undetected in other structurally complex and tectonically active regions globally, and that significant seismic hazards can be overlooked.